Studies of ammonia and bicarbonate transport are being conducted in isolated, perfused thick ascending limbs and cortical collecting ducts from rats and rabbits. Ammonia is absorbed by thick ascending limbs. This probably results from passive NH4+ transport driven by the lumen positive transepithelial electrical potential difference. Bicarbonate is actively absorbed by thick ascending limbs of rats. This results from secondary active secretion of protons, probably mediated by sodium-proton exchange across the apical membrane. Active bicarbonate secretion in the cortical collecting duct is stimulated by treatment of the experimental animals with deoxycorticosterone. The stimulation of bicarbonate secretion is prevented by acid loading the animals. Bicarbonate secretion is dependent on the presence of chloride in the perfusate and bath. The rate of bicarbonate secretion is increased by a lumen-to-bath chloride gradient and is decreased by a bath-to-lumen chloride gradient. Bicarbonate secretion is not dependent on the presence of sodium in the solutions. The isolated perfused cortical collecting duct spontaneously secretes ammonia. This has been shown to be dependent on an acid pH disequilibrium in the lumen and probably involves diffusional entry of NH3.